The present invention relates to a camera and, more particularly, to a camera having an auto-focus function.
Cameras are known having a so-called auto-focus function which performs an automatic focussing control in accordance with the result of a focus detection which is conducted by a focus detector on the basis of lights reflected by a photographing object. Known focus detectors used on this type of cameras, however, are capable of performing focus detection only when the amount of defocus, i.e., the amount of deviation of an image-formed position of the object formed by the photographic lens from a predetermined focal position, falls within a predetermined range. In other words, known focus detectors cannot detect focus condition when the amount of defocus exceeds the predetermined range. To obviate this problem an art is proposed in, for example, Japanese Patent Unexamined Publication (KOKAI) No. 61-45212, in which the photographic lens is moved to a different position while detecting the focus condition when the focus detection is failed, and the detection of the infocus position is performed after the photographic lens has come into a region which enables the focus detection. This control method is generally referred to as "low-contrast scan".
FIG. 9(a) shows the state of movement, i.e., positions of the photographic lens during automatic focus adjusting operation in the low-contrast scan mode, while FIG. 9(b) shows the flow of a sequence of the low-contrast scan process. since the sequence control of the low-contrast scan mode is performed during movement of the photographic lens, FIG. 9(a) shows the lens position at each step of the low-contrast scan process shown in FIG. 9(b).
In these Figures, DFCA1 and DFCA2 represent defocus coverage ranges which enable correct focus detection at the respective focus detecting moments. These defocus coverage ranges are shown in terms of the ranges over which the photographic lens is drivable. Thus, the lens drivable range which determines the defocus coverage range changes in accordance with the movement of the photographic lens. A symbol A represents the position of the photographic lens on which in-focus state with respect to a photographing object is attained.
The low-contrast scan process for detecting a focus condition will be explained on an assumption that the focus detection is conducted during the movement of the photographic lens from the infinite end phorographing position .infin. of the lens to the closest end photographing position of the lens.
During the low-contrast scanning operation, when the in-focus position A is within the defocus coverage range DFCA1, a charge accumulating operation for focus detection is performed by a focus detection light-receiving means (CCD) in a step represented by INTEGRATION 1. Then, in a step represented by DATA DUMP 1, the charges accumulated in INTEGRATION 1 and representing the data concerning the Light image received from the object are delivered to a microcomputer (CPU). Then, in a step represented by CALCULATION 1, a focus detection calculation is performed by the CPU, whereby a defocus amount represented by DFN in FIG. 9(b) is determined. However, since the photographic lens moves even during execution of the steps INTEGRATION 1 to CALCULATION 1, the lens has been moved by an amount CTC from the lens position at the step INTEGRATION 1, when the defocus amount DFN is determined at the end of the step CALCULATION 1. Thus, the lens may have been moved beyound the in-focus position A. In such a case, a control is executed to stop the movement of the lens without delay after the completion of the step CALCULATION 1. In response to this control, the lens stops after making an over-run by a distance OVRN due to inertia of the auto-focus motor and other parts. When the photographic lens is stopped, steps INTEGRATION 2 and CALCULATION 2 are executed to perform the focus detection for the purpose of confirmation of the in-focus position A.
Thus, in the known system, the focus detection is performed again through execution of the steps INTEGRATION 2 and CALCULATION 2, thus confirming the in-focus position A. It is, however, conceivable that the in-focus position A cannot be found due to a fact that the defocus coverage range has been shifted to DFCA2 as a result of the lens movement during focus-detection and the over-run of the lens after the control. In such a case, the CPU judges that focus cannot be found int eh focus detecting operation performed int eh step CALCULATION 2. In such a case, it is impossible to obtain the in-focus position of the photographic lens. Focus detection failure may also be experienced due to repetition of the low-contrast scan operation, even if the in-focus position is within the defocus coverage range DFCA 2. Therefore, when the focus detection is judged as being impossible in the step CALCULATION 2, the amount of movement of the lens to the in-focus position A is calculated by making use of the result of the focus detection conducted through the step INTEGRATION 1 to CALCULATION 1, and a control is performed to reverse the photographic lens so as to avoid focus detection failure.
As has been explained, the known system has a drawback in that the lens may have been moved beyond an in-focus position when the calculation of the defocus amount is completed, because the moving speed of the photographic lens during low-contrast scan is set very high, so that the focus detecting operation has to be conducted again after the stop of the photographic lens, for the purpose of confirmation of the in-focus position. In addition, when the in-focus position is out of the defocus coverage range after the stop of the lens, the focus detection is judged as being impossible and, in response to this judgment, the control is executed to reverse the lens movement in accordance with the result of the calculation of the distance to the in-focus position on the basis of the result of the preceding focus detecting operation. In consequence, a long time is required for the focus detection, as well as for reverse driving of the lens to the in-focus position, after the stop of the photographic lens. In particular, a considerably long time is consumed for stopping and reversing the photographic lens to the in-focus position. For these reasons, the known auto-focus system could not be used with a full satisfaction.